The Oliver Hirschbiegel-directed drama will centre on the last two years of the Princess's life as it explores her alleged affair with heart surgeon Dr Hasnat Khan.

Watts said of her casting in February: "It is such an honour to be able to play this iconic role - Princess Diana was loved across the world, and I look forward to rising to the challenge of playing her on screen."

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Jessica Chastain was previously attached to play Diana in Caught in Flight, while a separate biopic - from producer Stephen Evans - is also in development with Charlize Theron and Carey Mulligan among the rumoured candidates to play Diana.